Today we retested the same load that we shot last week, except with thicker coatings, just to see if the coating thickness makes any difference.
Unless otherwise stated, all of today's loads used air-cooled wheelweight, 26 gr. WW760, CCI #41 primers, and COL set right at the jam point.
Last week I used a single layer of powder coat, so this week I used 2 layers. Dry shake with BB's, baked at 400F. The bullets began life at 0.246" so the double powder coat was about 0.0015" thick.
Umpteen coats of Hi-Tek until it was 0.003" thick. Instructions say bake at 385 - 400 F, but I've found that it's best to err on the cool side because overheating seems to make the coating brittle and flakey, so I baked at 360 F after each coat.
Umpteen coats of VHT rattle can epoxy on top of a urethane prime coat, until it was 0.003" thick. I didn't bake between coats but did attempt to oven cure at the very end, but as you can see the epoxy "melted." I've successfully baked VHT before so I suspect what happened this time was that I didn't let it air dry sufficiently and there was still solvent trapped inside which boiled and made a mess. I was discouraged so I gave up on VHT epoxy for the time being.
Umpteen coats of urethane, shake & dry method, and oven cured. In theory oven curing should make it harder and tougher but it's anybody's guess what the correct curing temperature is.
Also umpteen coats of urethane on some gas check bullets. The urethane turns amber after oven curing.
The coated bullets were lubed with HVR, nose sized just enough to make the front band 0.242", then push-thru sized 0.246".
One of my beefs with Hi-Tek is that adhesion is marginal. A couple of the Hi-Tek bullets flaked during the sizing process.
Today's target. The good news is that all the coated loads shot groups, not wild patterns, and the groups were pretty consistent. The bad news is that they were lousy groups.
It's fair to say that the extra thick coatings were a flop. Oh well, at least that question has been answered and we can move on to other questions about coatings.Mean Radius from best to worst, including last week's results:
0.55" -- 11 BHN thin Urethane & HVR
0.65" -- 11 BHN thin Epoxy & HVR
0.82" -- 11 BHN 1-coat Powder Coat & HVR
0.94" -- 11 BHN 2-coat Powder Coat & HVR
0.99" -- 25 BHN thin Urethane & HVR
1.04" -- 11 BHN thick Urethane, HVR, GC, @2757 fps
1.15" -- 11 BHN thick Urethane, HVR, GC, @2462 fps
1.20" -- 11 BHN thick Hi-Tek & HVR
1.26" -- 11 BHN thick Urethane & HVRToday's velocities from lowest to highest:
2445 -- thick Hi-Tek
2448 -- 2-coat Powder Coat
2460 -- thick Urethane
2462 -- thick Urethane GC
2757 -- thick Urethane GC & 28 gr. WW760Today's velocity variation from lowest to highest:
0.58% -- thick Urethane GC & 28 gr. WW760
1.20% -- 2-coat Powder Coat
1.22% -- thick Urethane
1.27% -- thick Hi-Tek
1.30% -- thick Urethane GCConclusions, Observations, and Questions
-- the thick coats hurt rather than helped. My best guess is that 1) a thin coat suffices to prevent melting and gas cutting and 2) the thicker the coat, more imbalanced the bullet is likely to become after sizing.
-- not much difference between 1-coat & 2-coat powder coat.
-- not sure what to make of the 2757 fps gas check loads, since this was the first time I'd tried coated GC and the first time I'd tried an air-cooled bullet at 2700+ in this gun. I want to repeat the GC test with a thin coat and hard bullet.
-- once again I give up on Hi-Tek. It's "OK" but not superior in any way, and adhesion is marginal.
-- once again I am underwhelmed with powder coat. It's OK but not superior in any way.
-- note that the coated GC bullet did not shoot any better than coated PB when loaded to 2400 fps. It may be that since the coating prevents melting and gas cutting that the GC serves no purpose. More testing required to know for sure.Things to Try Next Time:
-- go back to 2 or 3 thin coats of either urethane or VHT epoxy
-- see how fast I can push coated PB
-- retest GC bullet with thin coat & hard alloy
-- as time and money allows I would like to try more coatings, i.e. spar urethane because it is supposed to be more flexible, water based urethane, and 2-part epoxy automotive paints.
Next post will show borecam pics for today's loads.